The focus of this application is the interaction between peptides and MHC class I molecules and how that interaction shapes subsequent immune responses. This problem will be addressed using peptides whose sequences are based on known T-cell epitopes. These peptides will be improved by altering non-anchor amino-acids and testing for altered binding. These sequences will be altered both by synthesizing new peptides and by selecting improved epitopes from random libraries. Binding will be assessed by evaluation of both on and off rates using binding in vivo to live cells, and in vitro in cell lysates. In addition the thermostability of the peptide-MHC complex will be determined by X-ray crystallography. Afferent immune responses will be measured in normal, and T-cell receptor transgenic mice. Effectiveness will be assessed by changes in antigen specific precursor frequency. These experiments will have important consequences for immunotherapy of cancer, autoimmune disease and synthetic vaccine design.